The invention relates to improvements in presses for extracting water from a continuing traveling web such as a newly formed web in a paper machine, and particularly the invention relates to a structure for providing an extended press nip which applies a pressing force to a web for a longer continuous time than structures of the type conventionally used such as formed by the nip of opposed roll couples. The invention particularly relates to an improved structure and method for obtaining and squeezing more water from the web than heretofore possible and accomplishing this function without disruption of the web fibers to obtain the formation of an improved web.
In the copending application of Busker and Francik, Ser. No. 193,272, now U.S. Pat. No. 3,798,121, the principles and advantages of pressing a paper web for an extended period of time and the advantages thereof are discussed. In the present structure, the principles of an extended nip are utilized in a structure affording advantages over prior art arrangements. In prior art structures such as conventional opposed rolls, the pressing pressure applied to a web is applied suddenly as the web passes through the nip and suddenly released to be again applied at a succeeding nip. In high speed paper machines, the pressure is applied very suddenly and over a very short period of time, and it has been found that hydraulic pressures due to flow resistance build up within the web preventing the water from escaping. If the pressure is increased, the amount of water removed is not significantly increased because of the resistance of the water to escaping in the relatively short period of time. Further disadvantages are encountered in that web fiber disruption, commonly called "crushing", occurs if the water pressures build up too rapidly within the nip.
It is accordingly an important feature of the present invention to provide an improved extended nip press which applies pressures stepwise to the web so that the water within the web can flow out at an optimum rate to achieve maximum dewatering without fiber disruption.
As will be appreciated from the teachings of the disclosure, the features of the invention may be employed in the dewatering of other forms of webs than a paper web in a paper making machine. However, for convenience, a preferred embodiment of the invention will be described in the environment of a paper making machine which conventionally forms a web by depositing a slurry of pulp fibers on a traveling fourdrinier wire, transfers the web to a press section where the web passes through a number of press nips formed between roll couples, and the web then passes over a series of heated dryer drums and usually through a calender and then is wound on the roll. The present structure forms the entire press section and takes the place of other forms of press sections heretofore available. Many modifications can be made in this type of overall machine, as to the forming section, the press section, the dryer section, and the structure of the instant disclosure may be employed in pressing webs of various synthetic fibers.
The present invention relates to improvements for the press sections of a paper making machine. In such a machine the web usually arrives at the press section with about 80 percent web basis moisture (ratio of water to fiber plus water) and leaves the press section with approximately 60 percent moisture, with the remaining moisture having to be removed by thermal evaporation in the dryer section as the web passes over a series of heated dryer drums. Because of various inherent limitations in the operation of roll couples forming press nips for the press section in a conventional paper making machine, only a given amount of water can be removed in each nip and, therefore, in a conventional paper making machine, a series of three press nips are usually employed. It has been found impractical to attempt to remove a significant amount of additional water by increasing the number of press nips, although the further removal of water by pressing can greatly reduce the expense and size of the dryer section. It is estimated that if the water removed in the press section can be increased to decrease moisture from 60 percent to 50 percent, the length of the dryer section can be reduced by one-third. This is significant in a typical 3000 feet per minute newsprint machine which employs on the order of 100 dryer drums. The significance can be appreciated in considering that the dryer drums are each expensive to construct and to operate and require the provision of steam fittings and a supply of steam for each drum. The relative importance of the removal of water in the press section is further highlighted by the fact that one of the most important economic considerations in justifying a satisfactory return on investment in the operation of a paper making machine is to obtain the highest speed possible consistent with good paper formation and better pressing will shorten the necessary time in the dryer section and permit higher speeds.
It is accordingly an object of the present invention to provide an improvement in the press section of a paper machine which makes it possible to remove an increased amount of water in this press secton and makes it possible to provide a press section having only a single pressing nip of a unique elongated or extended nature which does not have the performance limitations of conventional roll couple presses and which requires far less space in terms of requirements as to the overall length of the press section. By increasing the amount of water removed from the web in the press section, increased speeds are possible with existing equipment, i.e., a given length of dryer section can operate at higher speeds since it is required to remove less water. Also, new equipment can be constructed requiring less machine length and expense.
The present invention employs a principle which may be referred to as the extended nip concept wherein the time the web is subjected to a pressing action is greatly extended, i.e., a single pressing is provided having a residence time which exceeds that of the time of the web in a number of conventional roll couple press nips. With the reduction to a single pressing operation, the compound effects of rewetting the web as it leaves a plurality of nips are avoided.
A factor which presently limits water removal from paper by mechanical web pressing is the flow property of water within the paper sheet. It has been found that other factors are not of dominant significance, for example, the effects of the moisture in the left which travels with the web are small. It has been found further that the length of time that the web is in the nip, in other words the residence in the nip, can have a significant effect in overcoming the difficulties created by the flow properties of the water within the sheet. It has also been found that merely by increasing the residence time of the web in the nip, the water content of the sheet coming out of the press can be decreased so that the web will have 46 percent dryness rather than 40 percent dryness with other variables remaining constant. As is evident, the residence time of a web in a conventional roll couple press nip is limited and can only be increased by decreasing the speed of travel of the web, or can be increased slightly by increasing the diameter of the press rolls, but these factors are indeed limiting. It has been found, for example, that by applying a 1300 pound per square inch pressure on a web for five minutes, a moisture level of less than 30 percent can be attained. Yet, under the dynamic short term mechanical pressing of a paper machine press section using roll couples, even with a plurality of nips, a great deal of effort is required to maintain moisture levels below 60 percent.
It has been found that significant losses in dryness occur at higher speeds and that a loss in dryness of over 5 percent is experienced in going from 300 feet per minute to 1000 feet per minute with typical press loadings in suction press. It has been found that a hydraulic pressure or wedge effect develops during the passage of the wet mat through the wet press nip. The hydraulic pressure that develops substracts from the applied load and reduces the mechanical compacting pressure. The result is a loss in dryness. As the machine speed increases, the compacting rates are higher, resulting in higher hydraulic pressures within the paper mat. These hydraulic pressures react against the pressure of the rolls and prevent the moisture from being squeezed from the web. The exact value of hydraulic pressure is difficult to determine either by direct measure or analysis because of the space and speeds involved. It is believed, however, that hydraulic pressure predominately determines press performance on machines operating at high speeds. Accordingly, the instant invention relates to avoiding disadvantages encountered with high speed press nips of the conventional type used in most commercial applications today, and provides a substantial increase in residence time within a press nip to allow time for flow to occur withint the mat and for the hydraulic pressure to dissipate. The principles of extended nip or extended time pressing are further reviewed in the aforementioned application.